The invention relates to a ballast which is set up for parallel operation of discharge lamps and is provided, in particular, for surroundings where safety is important.
In potentially explosive surroundings, electrical equipment is operated, as a rule, in housings which are specifically suited to its application in order to prevent explosive or combustible gas-air mixtures from being ignited by the equipment. Nevertheless, the requirement exists that the equipment should not cause ignition even if it is held in inappropriate housings. In addition, no impermissible voltages are to occur.
Fluorescent lamps and gas discharge lamps generally have a limited service life. In the event of a fault or toward the end of their service life, the lamps may fail to start or no longer start reliably. In such instances, it has to be ensured that excessively high voltages, such as starting voltages, are not permanently present across the gas discharge lamp. On the other hand, a luminaire having a plurality of lamps is frequently intended to operate on one ballast so that, in the event of failure of one lamp, the luminaire as a whole can continue to illuminate, albeit at a reduced brightness. However, for economic reasons the lamps of a luminaire are to be powered as much as possible by a common ballast.
A circuit arrangement for operating a plurality of fluorescent lamps with a single ballast is known for this purpose from EP 558 772 A1. The ballast has an inverter which supplies a symmetrical AC voltage at a fixed frequency. Two fluorescent lamps are connected in parallel with the invertor, in each case via current-limiting series inductors. The two fluorescent lamps are connected to a monitoring device, which establishes whether the starting voltage is present at one of the fluorescent lamps for longer than a prescribed time interval T1. A switchover relay between the fluorescent lamps and the series inductors permits the two fluorescent lamps to be connected in parallel with one of the series inductors. If the monitoring device establishes a fault state in which the starting voltage is present at one of the fluorescent lamps for longer than the prescribed time interval, such an occurrence is interpreted as an instance of a fault. The switchover relay, which normally individually switches the two fluorescent lamps to be in series with the series inductors assigned to them, instead switches the fluorescent lamps to be parallel to one another and to one of the series inductors.
As a result, the non-ignited fluorescent lamp receives only the operating voltage of the ignited fluorescent lamp. However, this also means that the non-ignited fluorescent lamp is not off. Moreover, a mechanical relay is necessary.